1. Field of the Invention
The present invention relates to a tuner for a cable modem. More particularly, the present invention relates to a cable modem tuner that can input/output analog and digital signals via a CATV cable for processing.
2. Description of the Background Art
In the CATV, a coaxial cable is employed as the house drop line. The HFC (Hybrid Fiber/Coax) using an optical fiber for the trunk line is now being introduced.
The HFC is employed to provide data communication service with a broad band of several M bits per second to each domestic site. By employing the HFC, a high speed data line of the transmission rate of 30 M bits/second with a bandwidth of 6 MHz can be provided even by the 64 QAM (Quadrature Amplitude Modulation) system.
By using a cable modem for the above high speed data line, high speed data communication of 4 M bits/second to 27 M bits/second can be realized using an available channel of the CATV.
FIG. 17 is a schematic block diagram of a conventional cable modem tuner. Referring to FIG. 17, a cable modem tuner (referred to as "tuner" hereinafter) 100A is incorporated in a cable modem 117A. Tuner 100A is connected to a CATV station 118A via a CATV cable 114A, and also to a computer 116A external to cable modem 117A.
Tuner 100A includes a wideband amplifier 101, a first mixer circuit 102, a first intermediate frequency amplify input tuning circuit 103, a first intermediate frequency amplifier circuit 104, a first intermediate frequency output tuning circuit 105, a second mixer circuit 106, a first local oscillation circuit 107, a second local oscillation circuit 108, an up-stream circuit 109, a data terminal 110 connected to an external QPSK (Quadrature Phase Shift Keying) transmitter 115A, an input terminal 111, an output terminal 112, and a PLL (Phase Locked Loop) channel select circuit 113.
As to a CATV signal, the so-called up-stream signal and the so-called down-stream signal are operated at 5 MHz-42 MHz and 54 MHz-860 MHz, respectively. The up-stream signal is transmitted from input terminal 111 of tuner 100A towards CATV station 118A via cable 114A. The down-stream signal is transmitted from the end of CATV station 118A towards input terminal 111 of tuner 100A via cable 114A. The up-stream signal sent from tuner 100A is received by the data receiver of CATV station 118A (system operator) to be input to the center computer. As to the up-stream signal within cable modem 117A, a quadrature phase shift keyed data signal from, for example, QPSK transmitter 115 is applied to data terminal 110. This data signal is transmitted to CATV station 118A via data terminal 110, up-stream circuit 109, and input terminal 111.
As to the down-stream signal, the data signal received at CATV station 118A is modulated to, for example, 64 QAM and delivered on cable line 114A to cable modem 117A via input terminal 111. In cable modem 117, a desired signal is channel-tuned by tuner 100A with respect to the input data signal. Then, the signal obtained by channel-tuning is demodulated to 64 QAM by a circuit not shown in cable modem 117A. The demodulated signal is subjected to MPEG reproduction and then processed by a CPU (microcomputer) not shown. The processed signal is applied to TV monitor 116A.
The process of a down-stream signal within tuner 100A is set forth in the following. The down-stream signal input via input terminal 111 passes through wideband amplifier 101 and then converted into a first intermediate frequency (=950 MHz) by first mixer circuit 102 and first local oscillation circuit 107. Channel-tuning is carried out for the down-stream signal by PLL-controlling first local oscillation circuit 107 with a microcomputer not shown of PPL channel select circuit 113. The first IF signal (IF is the abbreviation of intermediate frequency) obtained by converting the down-stream signal into the first intermediate frequency is tuned by first intermediate frequency input tuning circuit 103 and then amplified by first intermediate frequency amplifier circuit 104. The amplified signal is channel-tuned by second intermediate frequency output tuning circuit 105 and then output to second mixer circuit 106. The IF signal having the first intermediate frequency is converted into a second IF signal by second local oscillation circuit 108 connected to second mixer circuit 106. In general, 44 MHz is used for the second intermediate frequency. The second IF signal obtained by conversion is output from output terminal 112. Here, second local oscillation circuit 108 is PLL-controlled by PLL channel select circuit 113, similar to first local oscillation circuit 107. The second IF signal output external of tuner 100A from output terminal 112 is converted into a baseband signal of 5 MHz by a circuit not shown in cable modem 117A. This signal is further A/D converted and demodulated according 60 QAM. The demodulated signal is subjected to the MPEG process to be output from cable modem 117A as a data signal.
Since tuner 100A constantly attains a standby state, low power consumption is required. Also, interference between each circuit is required in the double conversion type tuner 100A of FIG. 17. For this purpose, a casing design of an electrically strictly shielded structure must be provided for tuner 100A. Furthermore, a chassis design must be provided in which each circuit is spaced apart and in which interference is reduced. This is a bottleneck in reducing the size of tuner 100A. Local spurious disturbance easily occurs caused by interference between each local oscillation circuit of tuner 100A to result in communication error. It was therefore not possible to incorporate the circuit that converts the second IF signal into a baseband signal in the same casing where tuner 100A is accommodated.
In the so-called set top box mounted as a receiver on a television set, tuners for a digital signal and an analog signal are provided for respective channel-tuning. This double provision of the same type of tuners for the set top box results in redundant circuitry, and becomes a bottleneck in reducing the size and cost of the set top box.